Dianic Wicca is one of the many traditions within the larger Wiccan faith, and it can sometimes be a controversial one as well. If you’re not familiar with the concept of Wiccan traditions, you can get up to speed on that first.
Founded in 1971 by Zsuzsanna Budapest, Dianic Wicca is named for the Roman goddess Diana (though the tradition doesn’t worship Her exclusively). The defining quality of Dianic Wicca is it’s feminist approach to spirituality. Unlike other trads that have their own blends of male and female deities, Dianics solely work with various forms of the Goddess. Covens are female-only also.
The main reason why Dianic Wicca is a little controversial is that it very strictly focuses on Goddess-worship which is significant departure from the balanced male/female orientation of original Gardnerian Wicca. Some feel that it shouldn’t use the name Wicca because it is not following the fundamental tenets of the path.
Gender issues aside, Dianic Wicca is much like any other tradition. It is a blended mix of traditional British Wicca along with some additional forms of folk magic and Italian spirituality (as found in the book Aradia, by Charles Leland). There is a strong feminist leaning to all rituals within Dianic Wicca. Seasonal cycles of the earth are revered, as are all cycles and passage rites for girls and women. Political action for women’s rights is also a big part of many Dianic practices, though on average these groups are less political than the Reclaiming tradition.
One of Budapest’s books, “The Holy Book of Women’s Mysteries” is a good foundational text if you wanted to understand this Goddess path. You can also find more information about the Dianic path through organizations like the Circle of Aradia and the Temple of Diana.